A wide variety of block copolymers are prepared anionically, for example, by the polymerization of a single monomer to completion, utilizing organolithium compounds as polymerization initiators. The polymer chain produced has at least one active or living end, to provide a so-called "living polymer". Once the living polymer has been prepared, a second monomer is added and polymerization initiated by the living polymer to provide a block copolymer which, for practical purposes, consists of homopolymer segments. Difunctional initiators having two, or polyfunctional initiators having three or more, polymerization initiating sites, such as a polylithium compound, can be employed to prepare a variety of block copolymer structures. Many of such polymers are useful for a variety of purposes such as oxidatively-resistant thermoplastic elastomers, membranes, and thickeners or surfactants in water solutions.
Employing known anionic polymerization process techniques for block polymer preparations effectively excludes a wide variety of monomers, and their benefits, from incorporation into block copolymers.
For example, block copolymers having an ABA configuration are often prepared to have end, or A, blocks of an aromatic monomer block such as styrene, .alpha.-methylstyrene, or mixtures thereof; and the center, or B, block to have an elastomeric polymer block of an unsaturated diene such as butadiene, isoprene, or mixtures thereof. Such block polymers are readily prepared by polymerizing the center block first and then adding a second monomer to form the end blocks. However, the above technique is inconvenient for making triblock copolymers having butadiene end blocks and a styrene center block, and very difficult, because of the viscosity, for making triblock copolymers having styrene ends blocks and center blocks of polyvinylpyridines.
Poly(4-vinylpyridine) is not soluble in most solvents suitable for anionic polymerization. It can be made in pyridine, but styrene will not polymerize anionically in the presence of pyridine. It is possible to polymerize 2-vinylpyridine in a solvent such as benzene and then add styrene to form the end blocks, but the living styrene anions attack the pyridine segments and terminate or crosslink the chain. It is possible to make triblock polymers having polybutadiene as the center block by first polymerizing the center diene block and then adding styrene. If the 1,4-configuration of the polydiene is to be maintained, the polymerizing medium must be strictly non-polar. However, this leads to very high viscosities because of association of the polydienyl lithium ends.
In order to make triblock polymers having polymerized aromatic monomer end blocks and a polyvinylpyridine center block, it is necessary to make a diblock polymer first and then couple it. Conventional coupling reactions for such triblock polymers are generally effective only if the molecular weight of the first polymer chain is low, i.e., between about 10,000 to 20,000. Although not limited by theory, it is postulated that a proton transfer side reaction occurs in the coupling reaction. Because of the smaller number of living ends and the slower rates of reaction inherent with higher molecular weights, coupling of higher molecular weight polymers becomes limited in such conventional techniques. Thus, conventional anionic polymerization techniques limit the types of monomers which can be employed in preparing such types of block copolymers.
It would be advantageous if there were available an improved process for the preparation of block copolymers.
It would also be desirable if there were available an improved process for the preparation of block copolymers which would permit the inclusion of monomers which are generally difficult for anionic polymerization.